Steel alloys



Patented Feb. 27, 1945 STEEL ALLOYS Russell H. McCarroll and GostaVennerholm,

Dearborn, Mich., asslgnors to Ford Motor Company, Dean-born, Mich., acorporation of Delaware No Drawing. Application October 8, 1941, SerialNo. 414,210

6 Claims. "(01. 75-123) This invention relates to a new and improvedsteel alloy and heat treatment therefor, and to castings made andtreated in accordance therewith.

Much progress has been made in recent years in the development ofso-called cast alloy steels which have made it possible to produce manypreviously forged steel parts as castings, thus greatly reducing thecost of manufacture of such parts. However, the use of the castingprocess has been limited in many cases because some steels, while havingotherwise suitable physical properties, leave much to be desired incasting properties.

One of the principal objects of the invention is to provide a steelalloy that can be used for casting automotive and other parts previouslyrequiring fabrication by forging or other relatively expensive methods.

Another object of the invention is to provide a steel alloy of a desiredfairly high carbon content and heat treatment therefor capable ofproducing uniformly stronger and sounder steel I Per cent Carbon0.75-1.20

Silicon 1.00-1.50

Manganese 0.50-1.00

Molybdenum 1 0.15-0.35

1 Optional.

and the balance substantially iron (small amounts of phosphorus andsulfur, of course, being present). r

In practice, for certain mechanical parts, it is preferred to maintainthe composition within the following limits:

11 Per cent Carbon 0.90-1.10 Silicon 1.20-1.40 Manganese 0.60-0.80Molybdenum 1 0.15-0.25 Phosphorus 1 0.087

1 0 tional. 1 aximum.

casting than was heretofore attainable in castings having a somewhatsimilar carbon content.

Another object of the invention is to provide a steel alloy castinghaving an approximate eutectoid composition and in which the carbon inthe casting is predominantly in the combined form.

Still another object of the invention is to provide a steel alloy offairly high carbon content having casting properties improved by arelatively high silicon content, and containing carbon, silicon andmanganese in such proportions as to produce an approximate eutectoidcomposition thereby reducing the tendency to hot tear and to producedendrites and lines of weakness in the casting. A further object of theinvention is to make possible the production of cast steel articleshaving more satisfactory physical properties, such as increased tensilestrength, elasticity, elongation, machinability, etc., than could beobtained with prior known steel alloys.

A still further object of the invention is to provide a novel heattreatment for the steel alloy which will prevent grain growthand breakup the laminated carbides forming the perlite into small roundedindividual particles or spheroids.

Other and further objects of the invention will be apparent from thefollowing description:

Among the many parts which may be cast from the present alloy, insteadof being forged. may be mentioned rear axle housings for trucks. furrowwheels, etc.

The various alloys comprising the present invention lie within thefollowing range:

and the balance substantially iron.'

As a specific example of a steel composition within the limits ofFormulae I and 11 above, it has been found that desirable results incastin can be obtained with a steel having a specific analysissubstantially as follows:

m Per cent Carbon 1.00

Silicon 1.30

Manganese 0.70

Molybdenum 1 0.20

1 Optional.

and the balance substantially iron.

Another highly desirable composition falling within the broad range ofFormula I is as follows:

Per cent Carbon 0.80-1.00 Silicon 1.20-1.40 Manganese ".1--- 0.50-0.70Molybdenum 1 0.15-0.25 Phosphorus g 1 0.06 Sulfur 1 0.06

0 tional. 1 aximum.

and the balance substantially iron.

A typical analysis falling within Formula IV and in which the carboncontent is less than 1% is as follows:

and the balance substantially iron.

It is to be noted from the foregoing formulae that the use of molybdenumis optional and may be omitted from the composition if desired.

In the above examples and ranges of the present steel alloy, the maximumspecified range of carbon content is from 0.75% to 1.20%. Heretofore ithas been considered advisable in actual practice to form articles orparts in this carbon range by the conventional forging or similarprocesses, because a casting process for such steel was consideredcommercially impracticable. It is, therefore, an important considerationthat the improved casting properties of the present steel alloy makescommercially practicable the extension of steel casting methods to alarge and important class of high grade steel parts which previouslyrequired more expensive and elaborate methods for successfulmanufacture.

The above examples illustrating the composition of the present steelalloy are characterized by their relatively high silicon content andalso a relatively increased manganese. content, although to a lesserextent. It has been found that if the above specified ranges andspecific examples are followed an approximate eutectoid composition ismaintained which reduces the tendency to precipitate material formingdendrites and planes of weakness, and, therefore, the casting propertiesof the steel are materially improved. The essential thing is that withthe ratios disclosed there is a substantial balance of elements assuringan approximate eutectoid composition in the carbon range of 0.75% to1.20%, and this is accomplished mainly because of the relatively highsilicon content, there being a greater percentage of silicon thancarbon.

Although the present alloy, so far as the carbon content is concerned,falls within the range which is generally termed tool steel, the effectof the alloying elements, especially high silicon and relatively highmanganese, give the alloy exceptionally good casting properties.Especially noticeable is the high hot metal strength which reduces thetendency to hot tear and also the tendency to form dendrites which wouldresult in a weakening of the casting. Probably the greatest objection tothe prior materials lies in the fact that the dendrites, hot tears orplanes of weakness, do not always show up during machining or assembly,or, even, when given severe production tests, for example, a drop teston a truck rear axle housing.

It will be seen from the ranges and specific compositions set forthherein, that, as a general proposition, the sum of the amounts ofsilicon and manganese is approximately twice that of the carbon. It willalso be noted that the amount of silicon is approximately twice that ofthe manganese. Thus, there is a more or less definiterelationshipbetween the amounts of silicon, manganese and carbonentering into the composition. As a specific -omparison, and consideringFormula III as illustrative, it will be noted that the manganese is0.70%, the silicon is 1.30% and that their sum makes a total of 2.00%.When this sum is compared with that of the carbon content, namely,1.00%, it is apparent that the total amount of manganese and silicon istwice that of the carbon. It will also be noted that the 1.30% siliconis approximately twice that of the manganese. which is 0.70%. However,it will be understood that reasonable variations in the proportions andin the relationships set forth can be made without destroying thecasting advantages and physical characteristics of the steel alloy.

Castings made .of the steel alloys disclosed herein are preferably heattreated in a continuous type furnace (not requiring illustration herein)differing from the conventional type mainly in the respect that thefurnace is made in two sections, thereby making it possible to provide aheat treatment with an interposed air quench as described below.

In accordance with the present invention, the castings are introducedinto the first section of the furnace on a conveyor and are graduallybrought up to a temperature of about 1725 F., the time required to reachthis temperature being about two hours. The castings are permitted toremain or soak in the furnace at this temperature for about one hour,and are then removed from the first section of the furnace and rapidlyair cooled to below the critical temperature, that is, to about 1200 F.,the air cooling or quenching'being effected in the relatively shortperiod of approximately fifteen minutes. The effect of a rapid coolingupon the structure at this high temperature is very pronounced, in thatit not only produces a pronounced equalization of the cast structure,but also prevents grain growth.

After the rapid air quenching, the conveyor carries the castings intothe second portion of the furnace where they are reheated toapproximately 1400 F. in one hour, and held between 1400 F. and 1460" F.for a period of about two and one-half hours, after which thetemperature of the castings is lowered to approximately 1300 F. at therate of 25 F. per hour. The castings are then permitted to cool in airfrom 1300 F. to atmospheric temperature.

The second portion of the heat treatment cycle. namely, that in whichthe metal is reheated to 1400" F. and held at about such temperature fora substantial period of time, is of vital importance as thespheroidization takes place during this period. The spheroidizationprocess consists of a breaking up of the laminated carbides forming theperlite into small rounded individual particles or pheroids.

The foregoing heat treatment produces a fine substantially uniformdistribution of the carbides resulting in a much tougher and strongermaterial than would be the case where a coarse lamellar perlitepredominates with each lamella constituting a plane of weakness. Stateddifferently, the present heat treatment provides a fine spheroidizedmatrix characterized by a complete absence of primary graphite and withany tempercarbon present of almost sub-microscopic size.

The present alloy has superior casting qualities relative to other steelalloys 'of both low carbon, say .30% to .60% and higher carbon, say1.50% or thereabout. In addition to the improved foundry quality of themetal over other cast alloys, the present metal can be satisfactorilyheat treated, as specified, to obtain a metal ranging in tensilestrength from about 65,000 to 120,000 pounds per square inch, withelongation properties of from 9% to 20% and with a modulus of elasticityof over 29,000,000 pounds per square inch. The high modulus ofelasticity is of utmost importance as other cast alloys of a similartype have a modulus of from as low as 23,000,000 to a maximum of28,000,000 pounds per square inch.

Because of the approximate eutectoid composition of the present steelalloy, it is possible to take the utmost advantage of the carbon contentin heat treatment without either precipitating quantities of freecementite particles, which cause increase in hardness and brittleness,or, when annealing, without producing quantities of free temper carbonwhich also weakens the material.

It is also true that where lower tensile strength but higher ductilityis desired, the material reacts remarkably well to annealing treatmentsof the short cycle malleable type resulting in an extremely tough andductile casting with elongation in excess of 20% and a tensile strengthof over 65,000 pounds per square inch. Also by applying a so-calledspheroidizing treatment comprising a prolonged heating near the criticaltemperature followed by slow cooling, a material of exceptionally hightensile strength of over 110,000 pounds per square inch and anelongation of about is obtainable, thus making it possible to replacelarge numbers of expensive low carbon steel castings as well as forgingsat a considerable saving and improvement in properties.

It will be noted that the present invention is characterized by the factthat a steel casting is produced, as contradistinguished from malleableiron, such for example as exhibited in the patent to Edmunds No.2,069,717, dated February 2, 1937. For example, steel castings made inaccordance with the present invention are characterized by a higherelastic limit; greater machinability, tensile strength, freedom fromdendritic planes of weakness, etc.

This application is a continuation-in-part of co-pending applicationSerial No. 351,123, filed August 3, 1940, and entitled Steel.

While certain specific compositions and ranges of the alloy have beenset forth herein, and while specific temperatures and times have beengiven for the heat treatment of the alloy, it will be understood thatreasonable departures may be made from both the composition and heattreatment without departing from the spirit or scope of the invention.

We claim:

l. A steel alloy casting having a matrix consisting of spheroiclizedpearlite and having substantially all of its free carbon in the form ofminute particles of temper carbon embedded in said matrix andsubstantially uniformly distributed throughout said matrix, said castingcomprising carbon about 1.0%; silicon about 1.30%; manganese about0.70%; and the balance being substantially iron, the ratio of the totalamount of silicon and manganese to that of the carbon, and of the amountof silicon to that of the manganese being so proportioned that thematrix as cast is substantially entirely pearlitic and is characterizedby being free from hot tears, dendrites and planes of weamess.

2. A steel alloy casting having a matrix consisting of spheroidizedpearlite and having substantially all of its free carbon in the form ofminute particles of temper carbon embedded in said matrix andsubstantially uniformly dis-I tributed throughout said matrix, saidcasting comprising: carbon ranging from .80% to 1.00%; silicon rangingfrom 1.20% to 1.40%; manganese ranging from .5% to .7%; and the balancebeing substantially iron, the ratio of thetotal amount of silicon andmanganese to that of the carbon, and of the amount of silicon to that ofthe manganese being so proportioned that the matrix as cast issubstantially entirely pearlitic and is characterized by being from hottears, dendrites and planes of weakness.

3. A steel alloy casting having a matrix consisting of spheroidizedpearlite and having substantially all of its free carbon in the form ofminute particles of temper carbon embedded in said matrix andsubstantially uniformly distributed throughout said matrix, said castingcomprising: carbonranging from .80% to 1.00%; silicon ranging from 1.20%to 1.40% manganese ranging from .5% to .'7%; molybdenum ranging from.15% to 25%; and the balance being substantially iron, the amount ofsilicon being substantially twice that of the manganese and the ratio ofthe total amount of silicon and manganese to that of the carbon being soproportioned that the matrix as cast. is substantially entirely peariticand is characterized by being free from hot tears, dendrites and planesof weakness.

4. A steel alloy casting having a. matrix consisting of spheroidizedpearlite and having substantially all of its free carbon in the formofminute particles of temper carbon embedded in said matrix andsubstantially uniformly distributed throughout said matrix, said castingcomprising: carbon about .95%; silicon about 1.35%; manganese about0.50%; and the balance being substantially iron, the total amount ofsilicon and manganese being approximately twice that of the carbon andthe amount of silicon relative to the amount of manganese being soproportioned that the matrix as cast is substantially entirely pearliticand is characterized by being free from hot tears, dendrites and planesof weakness.

5. A steel alloy casting having a matrix consisting of spheroidizedpearlite and having substantially all of its free carbon in the form ofminute particles of temper carbon embedded in said matrix andsubstantially uniformly distributed throughout said matrix, said castingcomprising: carbon about .95%; silicon about 1.35%; manganese about.50%; molybdenum about 0.17%; phosphorus about 0.06%; sulfur about0.06%; and the balance being substantially iron, the total amount ofsilicon and manganese being approximately twice that of the carbon andthe amount of silicon relative to the amount of manganese being soproportioned that the matrix as cast is substantially entirely pearliticand is characterized by being free from hot tears, dendrites and planesof weakness.

6. A steel alloy casting having a matrix consisting of spheroidizedpearlite and having substantially all of its free carbon in the form ofminute particles of temper carbon embedded in said matrix andsubstantially uniformly distributed throughout said matrix, said castingcomprising: carbon ranging from 0.75% to 1.00%; silicon ranging from1.00% to 1.50%; manganese ranging from 0.50% to 1.00%; molybdenumranging from 0.15% to 0.35%; and the balance being substantially iron,the ratio of the total amount of silicon and manganese to that of thecarbon, and of the amount of silicon to that 01' the manganese being soproportioned that the matrix as cast is substantially entirely pearliticand is characterized by being free from hot tears, dendrites and planesof weakness.

RUSSELL H. MCCARROLL. GOSTA VENNERHOLM.

